The simian immunodeficiency virus (SIV) infection of rhesus macaques provides one of the best available animal model to investigate the pathogenesis of AIDS in humans and to test and develop vaccine strategies for protection against infection and alteration in disease progression. The pathogenesis of SIV induced disease in rhesus macaques closely resembles AIDS in humans. Recent studies showing that macrophage-tropic strains of HIV-1 are the viruses transmitted and present early after infection have suggested that vaccine approaches utilize macrophage- tropic viruses. An attenuated macrophage-tropic recombinant SIV was used to immunize five rhesus macaques, a rapid and vigorous immune response developed. Challenge of two of these animals showed that they were resistant to infection with a heterologous lymphocyte-tropic SIV. The particular role of macrophage-tropic virus in protective immune responses will be investigated in this project by the construction of macrophage- tropic recombinants of SIV for analyses in rhesus macaques. Another important aspect of vaccine development includes the testing of candidates in an animal model. However, there are biological and molecular differences between HIV-1 and SIV that limit the utility of the SIV model for vaccines trials against HIV-1. The envelope protein of HIV-1 differs significantly from that of SIVmac in the immunological responses of the host to this viral protein as well as in the determinants for virus neutralization. A newly evolving animal model utilizing SIVmac/HIV-1 chimeric viruses that contain the envelope gene of HIV-1 (as well as other regulatory proteins) can potentially overcome the problem with the SIV model. Infection of monkeys with such chimeric viruses have been reported. The goal of this project is to construct and characterize infectious macrophage-tropic molecular recombinants of SIVmac and SIVmac/HIV-1 chimeric viruses to use in the macaque model for vaccine development and testing. A yeast based recombination system will be used to facilitate the rapid generation of infectious recombinant viruses. A yeast artificial chromosome (YAC) containing infectious molecular clones of SIVmac have been made in both T lymphocyte-tropic as well as macrophage- tropic strains of SIVmac to be used in these studies; these viruses will be used to examine the mechanism of protection of macrophage-tropic virus and the role of the nef gene in virus replication and attenuation in macrophages. Chimeric SIV/HIV viruses will be optimized for in vitro replication by supplying the SIV tat gene to increase gene expression and modifications in the LTR will also be examined for their role in increased virus replication. The optimized chimeric SIV/HIV virus will be used to construct additional viruses that contain the env genes of HIV-1 isolates that are being used as vaccine candidates.